Where does Apple install the "main" SSD to?

Something I have been curious about for quite some time now: when you order a CTO SSD in your iMac (not a HDD+SSD, combination, but an SSD only) - where do they install it? Does it go into the 3.5" HDD slot, or do they actually install it into the smaller SSD slot behind? My google-fu could not give me an answer, maybe someone here has some information?

MacRumors attracts a broad audience
of both consumers and professionals interested in
the latest technologies and products. We also boast an active community focused on
purchasing decisions and technical aspects of the iPhone, iPod, iPad, and Mac platforms.